In railways or press-controlling fields where a high fail-safe characteristic is required, it is indispensable to use an apparatus or equipment having such a fail-safe property that when a short circuit, a breaking or the like takes place the circuit is switched over to the safe side without fail. Therefore, when a load which is a final object to be controlled, for example, a motor or a relay, is driven, the fail-safe characteristic is similarly required.
If a switch circuit for driving a load is directly controlled by an input signal in view of this fail-safe property, a trouble in the switch circuit results in malfunction of the load. Therefore, according to the conventional technique, the load cannot be directly controlled by an input signal, and a signal-receiving system has to be interposed between the input signal, and the load so as to maintain the fall-safe effect. For example, in a conventional load-driving switch circuit, in order to ensure maintenance of the fail-safe effect, there is provided a direct current relay having a silver-carbon type contact, in which a fusion trouble seldom occurs, including a signal-receiving circuit comprising an amplifier, a transformer and a rectifying circuit interposed between this direct current relay and an input terminal for a control signal to amplify an alternating current signal Si applied to the input terminal through the transformer. The amplified signal is supplied to the rectifying circuit to convert it to a direct current signal, and the direct current relay is excited by the converted direct current signal to drive the load indirectly through this contact.
Thus, in the conventional load-driving switch circuit, the fail-safe effect is attained by utilizing the transformer-connected amplifier (amplification by an alternating current operation) and the direct current relay. According to this method, however, it is impossible to control the load directly by an input signal. Furthermore, since a transistor constituting the signal-receiving transistor is used in an unsaturated region because of its function, the circuit efficiency is low, and moreover, the structure between the input signal and the load is complicated and the connection has to be effected through a relay contact.
Furthermore, it is known that a relay is used for a safety apparatus used in the field of pressing machines. This safety apparatus is constructed so that when a hand of an operator is placed on a bolster plate, a slide is inhibited from falling down, and the relay is used for detecting an output of a sensor of the safety apparatus (the sensor detects the presence of the operator's hand on the bolster plate or in the vicinity thereof). This output relay is directly driven by a collector of a transistor without passage through a transformer. However, in order to make it always possible to inspect whether or not the relay is turned on even in the absence of an instruction of turning on the contact output because of trouble in the driving transistor or fusion trouble of the contact, inspection is ordinarily performed by using, for example, a switch for reducing the relay-driving input to zero.
However, this inspection becomes insignificant if a trouble takes place during inspecting, and in order to ensure safety, the inspection should always be performed by an operator.